1. Field of the Invention
The invention relates to a process for the selective halogenation of oligophenyls in the 4,4'-position in the presence of zeolites having pore widths of at least 5 .ANG. and also in the presence of methylene chloride.
Oligophenyls which are halogenated in the p-position on both sides, such as 4,4'-dichloro- and 4,4'-dibromo-diphenyl or -terphenyl, are claiming great industrial interest, for example as intermediate products for plastics of high heat resistance, such as polyphenylene sulphide (compare Japanese Patent Application 61/231,030 and U.S. Pat. No. 3,396,110 [C.A. 69, P 60 564 w]).
2. Description of the Related Art
Conventional chlorination of diphenyl in the presence of Lewis acids leads to non-selective random substitution, the 4,4'-derivative not being the preferred derivative. Chlorination at 100.degree. C. in the presence of 2.5% by weight of FeCl.sub.3 thus produces a selectivity of only 8% for the 4,4'-dichloro isomer; the content of trichlorobiphenyl is remarkably high, at 15%, polychlorinated biphenyls, as is known, belonging to highly toxic classes of substances. According to the information in U.S. Pat. No. 1,946,040, U.S. Pat. No. 3,226,447 and British Patent Specification 1,153,746, the addition of a sulphur compound during the chlorination of benzenes is said to increase the selectivity in favour of para-substitution. As the comparison example described below shows, however, in such a chlorination of biphenyl by addition of 2.5% by weight of thiophene, in addition to 2.5% by weight of FeCl.sub.3, the selectivity is increased only insignificantly in favour of 4,4'-dichlorobiphenyl, whereas polychlorinated biphenyls still make up a considerable proportion of the reaction product, at more than 9% by weight. This fact indicates that biphenyl and benzene are comparable to only a very limited degree.
The chlorination of p-terphenyl in the presence of iron and chloroform leads to a random mixture of o-and p-monochloro- and o,p'- and p,p'-dichloro-terphenyls, which could be separated only with great technical expenditure because of the sparingly soluble nature of the p-isomers (Bull. Soc. Chim. France 1968 (10), 4255-58 [C.A. 70 (1969), 57 316 g]).
The use of Lewis acids in practice furthermore as is known leads to increased corrosion problems and to a more difficult working up and disposal.